1. THE FIELD OF THE INVENTION
The present invention relates to a method for the selective methylation of the hydroxy group at the 6-position of erythromycin A derivatives.
2. DESCRIPTION OF THE PRIOR ART
6-0-Methylerythromycins are useful as anti-bacterial agents or intermediates for synthesis of the anti-bacterial agents. For example, 6-0-methylerythromycin A is not only stable in the acidic conditions but also has a strong anti-bacterial activity when compared with erythromycin A. Especially, this compound shows an excellent effect for treatment of infections by oral administration, and therefore it is a useful anti-bacterial agent.
There is known in the past a method for methylating the hydroxy group at the 6-position of the erythromycin A derivative which comprises substituting the hydrogen atom of the hydroxy group at the 2'-position and the methyl group of the dimethylamino group at the 3'-position of erythromycin A derivative by benzyloxycarbonyl groups, and reacting the resulting compound with a methylating agent (U.S. Pat. No. 4,331,803). However, since erythromycin A has many hydroxy groups, there are obtained various kinds of erythromycin A derivatives which are methylated at hydroxy groups at any other than the 6-position as the by-products besides the desired 6-0-methylerythromycin A derivative by the above-mentioned known method. Accordingly, this method requires the complicated procedure for purification of the 6-0-methylerythromycin A derivative, and the yield of said derivative is low.